When a ship is moored alongside a wharf or a quay in a harbour, mooring ropes anchoring the ship must be properly tensioned so as to hold the ship in an appropriate position. If no effort is made to maintain the mooring ropes in correct tension, a hazardous situation might arise for the reason that the mooring ropes will become subjected to greater forces due to the tendency of the ship to move relative to the wharf or quay. There are a number of factors that may make the ship to move relative to the wharf or quay. These factors can be, for example, variations of the level of water surface due to the cyclic tidal changes and variations of the displacement of the ship due to cargo loading and/or unloading. These factors will cause the ship to vary its altitude with respect to the wharf or quay, and hence will vary the tension of the mooring ropes of a given length between ship and the wharf or quay. Furthermore, the ship might be rocked or rolled by waves or wind to induce a fluctuating tension in the mooring ropes. In a situation in which the movements have great amplitudes, the mooring ropes might fail, resulting in a danger to personnel in the near area and a risk of damages to the ship.
Publication EP0676365 discloses a winch having at least one winding drum that is connected to an electrical drive via a gearbox. The electrical drive is an asynchronous alternating current motor connected to a speed control device and fitted with a brake device. The speed control has a speed indicator for detecting an existing rotational speed. The speed control device is coordinated by a control unit which may be for example a programmable controller taking the detected rotational speed and a target value of the rotational speed as inputs. A critical part of the winch described above is the speed indicator that is susceptible to hard weather conditions especially when the winch is being used as an open deck machinery of a ship.